8.10.2.5 Key Cache Block Size

It is possible to specify the size of the block buffers for an
individual key cache using the
key_cache_block_size
variable. This permits tuning of the performance of I/O
operations for index files.

The best performance for I/O operations is achieved when the
size of read buffers is equal to the size of the native
operating system I/O buffers. But setting the size of key
nodes equal to the size of the I/O buffer does not always
ensure the best overall performance. When reading the big leaf
nodes, the server pulls in a lot of unnecessary data,
effectively preventing reading other leaf nodes.

To control the size of blocks in the .MYI
index file of MyISAM tables, use the
--myisam-block-size option at
server startup.